Ultrasonic penetrant method of flaw detection



United States Patent Office 2,856,538 Patented Oct. 14, 1958 ULTRASONICPENETRANT METHOD OF FLAW DETECTION No Drawing. Application July 27, 1955Serial No. 524,805

11 Claims. (Cl. 250--71) Ohio, assignor to Cleveland, Uhio, acorporation This invention relates to improvements in penetrant methodsof detecting flaws having surface openings which may exist in parts andstructures (hereinafter referred to as test bodies). More particularlythe invention relates to improvements in such methods by the use ofhighfrequency vibrations, usually in the near ultrasonic range, i. e.,from approximately 10,000 cycles per second to 500,- 000 cycles persecond. However, one may often use vibrations down to the higher audiblefrequencies (i. e., to frequencies as low as 6,000 to 8,000 C. P. S.)and occasionally one way use vibrations in the high ultrasonic range, i.e., in excess of 500,000 C. P. S. The term sonic is usually understoodto be synonymous with audible, but as a term to designate the range offrequencies broadly contemplated by this invention, the term sonic willbe used hereinafter.

Ever since the introduction to the art of the fluorescent penetrantmethod of flaw detection disclosed in the United States patent to RobertC. Switzer, No. 2,259,400, for Flaw Detection, penetrant methods of flawdetection have become increasingly significant methods in the field ofnon-destructive testing. Essentially the method is limited to thedetection of cracks, tears, blow-holes, laps, and the like havingsurface openings, as contrasted with other methods such as X-ray,magnetic flux, reflected ultrasonic pulse, and like methods which aregenerally employed to locate flaws in the interior of the test body,although some, such as the magnetic flux method, are also frequentlyeffective, when they are operative to any appreciable degree, inlocating flaws having surface openings as well as purely internal flaws.The reason for the increasing use of the penetrant method of inspectionis that a minute surface discontinuity, especially at or near areas ofmaximum stress, is often likely to be the source of failure in servicewhereas purely internal flaws, unless quite gross or in a part designedwith a very small factor of safety, may be relatively harmless.

The penetrant methods of inspection, in general, comprise the steps ofapplying a penetrant liquid to the surface of the test body, allowingportions of the penetrant to enter the flaws, removing the remainingpenetrant from the surface of the body, allowing the penetrant retainedin the flaws to appear at or in flaw openings, and then inspecting thebody for the indication made on the surface by the retained penetrant.Where the penetrant is fluorescent and inspection is carried out underfluorescigenous radiations (e. g., black light) in accordance with theaforesaid Switzer process, very minute flaws are readily located. Sincethe introduction of the Switzer method, various and significantimprovements have been made, as in methods of removing the penetrantfrom the surface of the article without removing appreciable quantitiesfrom the flaws, in aiding the development of the flaw indication,increasing the fluorescent efiiciency of the penetrants, and the like.Also, various techniques in removing the penetrant from the surface anddeveloping subsequent flaw indications at or around the flaw openingshave permitted the use of non-fluorescent penetrants whose indicationsare observable in ordinary visible light.

With relatively few exceptions, such as in the U. S. patent to RichardA. Ward, No. 2,405,078 for Methods and Compositions for Locating SurfaceDiscontinuities," which teaches the art that a proper sequence ofheating and cooling the test body can aid the actual penetration of theflaws by the penetrant, those skilled in the art may seem to havegenerally overlooked the fact that, regardless of how the characteristicof the penetrant may be improved, or how precisely the penetrant may beremoved from the surface of the test body without removal from theflaws, or how efiectively the indication may be developed, the successof the operation of any of the penetrant methods of flaw detectiondepends upon getting the penetrant into the flaws and subsequentlyhaving retained penetrant appear at least at, .and generally preferablyaround, the flaw opening.

Actually, the interest in and development of aspects of penetrantinspection methods other than procedures to promote penetration andexpulsion of the penetrant has not been due to an oversight by the art.Rather, heretofore, there did not appear to be much of anything whichcould be done to improve penetration and expulsion of the penetrantbeyond careful formulation of the penetrant'to obtain optimumpenetrability with respect to the materials of the bodies to be testedand, of course, to thoroughly clean the test body of extraneous matterwhich might clog the flaw openings, as taught by the above Switzer andWard patents. Such apparent expedients as application of pressure,lowering the viscosity of the penetrant with solvent, et cetera, made noappreciable improvement in the aspect of the process. In the firstplace, lowering viscosity does not necessarily improve penetrability; inthe second place, the surface areas of the flaw openings to be locatedare usually very small; thus, enormous pressures, in terms of pounds persquare inch, become, at the flaw openings, very insignificant forces.

It is an object and advantage of this invention to improve thepenetration of liquid penetrants into flaws in penetrant inspectionmethods, and by the same forces, namely, sonic forces, to aid in theexpulsion of such retained penetrants to or around flaw openings.

A further object and advantage of this invention is to improve thesensitivity of penetrant inspection methods and to increase the capacityof existing penetrant inspection facilities by shortening the timerequired for penetration and expulsion.

Other objects and advantages of this invention will be apparent from thefollowing general and detailed disclosure and the appended claims.

As indicated above, for satisfactory and reliable detection of flaws ina test body it is necessary to remove any foreign matter which may blockor clog the flaw openings. As one procedure for effecting cleaning, theso-called ultrasonic procedure Was tested and found, in general, tooffer no particlar advantage over other methods except for small parts,which can be cleaned en masse in a tank of ultrasonically vibratedliquid. In ultrasonic cleaning the cleaning liquid is vibratedultrasonically either by a transducer submerged in the bath or by anultrasonically vibrated bottom and/ or walls of the vessel containingthe liquid. It has been proposed to submerge a suspected part in anultrasonically vibrated bath, not of the usual light hydrocarbon orchlorinated hydrocarbon employed for ultrasonic cleaning, but of apreferred fluorescent penetrant. Where the suspected part had a tightlyclosed cold shut extending to its surface, improved penetration wasobserved. This result is assumed to follow from two concepts. Oneconcept is that, with the high proportion of kerosene or like lighthydrocarbon in many fluorescent penetrants and the emulsifying agentoften present to render the penetrant self-emulsifying, the penetrantvehicle can itself function as a cleansing liquid. The second conceptis: that possibly one reason ultrasonic cleaning was not particularlyeffective in cleaning tightly closed flaws is because normal cleaningliquids may remove foreignmatter from such flaws but, in doing so, thecleaning liquids may remain in such flaws and become, themselves,foreign matter resisting penetration. and displacementby subsequentlyapplied fluorescent penetrants; by using thelfluorescent penetrantitself as the cleaning liquid, any; such. residual cleaning. agent wouldreveal itself in subsequent inspection under. fluorescigenousradiations, provided a sufficient volume of penetrant was retained inthe flaw to permit detectable amounts to creep to and appear at. or.around the flaw opening at the time of inspection. Unfortunately, exceptin the foregoing instances where the above postulates would have anopportunity: to be effective, ultrasonic cleaning or immersion in a.vibrating bath. of fluorescent penetrants offered no noticeableimprovement in ultimate results over those obtainable by standardprocedures of cleaning by any suitablemeans, application and removal ofpenetrant after allowance of: time for penetration and reappearance ofretainedpenetrant, and inspection. for. retained penetrantatandaroundflaw openings. 1

It;had also beennoted that when parts which had been subjected to.penetrant inspection methods were subsequently inspected. by reflectedultrasonic pulses to locate internal laminations and the like, suchultrasonic pulses did not noticeably promote the expulsion of thepenetrant from fiaws which had surface openings.

In view of the foregoing experiences, it was, therefore, with".considerable surprise that an ultrasonically driven magnetostrictivetransducer was observed (when it happended' to beremoved while stillbeing driven ultrasonieally-atabout 10,500 C. P. S. from a liquidhavingrather poor penetrating characteristics) to expel, nonetheless,liquid with considerable force from a minute fatigue crack thatwasapparently developing in theweld securing the face plate to the armatureof the transducer. Since the transducer with its incipient surfacedefect had only been immersed for a very short period of time, severalconclusions could be drawn from this chance observation: (1)The volumeof liquid expelled from the slight opening indicated that the voidbehind the opening was much larger than would be expected from the sizeof the opening. (2) The volume of liquid which had penetrated into thevoid through the opening was much greater than would be'expected to havepenetrated in the period of time the transducer was immersed if thepenetration was due solely to undisturbed'surface active forces, theforces presumably relied on in prior penetrant inspection methods. (3)The liquid was being expelled from the flaw at'a vastly greater ratethan the slowrate usually allowed form prior penetrant inspectionmethods. (4) The-liquid which had penetrated into the'void' hadsomehowhad its penetrating characteristic greatly improved. (5) Theflaw" in the transducer was behaving entirely differently-from' behaviorwhich would be expected from previous experiences in subjecting articleshaving surface flaws to sonic vibrations.

Initially the only tenable explanation for the above observed resultsappeared to be that, in the prior experiences-the articles subjected toultrasonic vibration probably did not have a natural frequencycorresponding to the'frequency of the sonic vibrations to which theywere subjected, whereas the frequencies imposed on the flawed transducerapproximated the natural frequency of the transducer, it being a generalrequirement of transducer designthat its mass and shape have a naturalfrequency approximating the'frequency to be imposed upon it.Furthertests-of the validity of the' foregoing explanation do notappearto support a conclusion that, inorder to improve the penetrabilityof a liquid penetrant into a flaw having a surface opening in a solidarticle and to drive the penetrant into and out of the flawultrasonically, the sonic frequencies must approximate the naturalfrequency of the article. Rather, it seems to be only necessary toselect a sonic frequency such that the article will absorb and vibratein response to, i. e., resonate with, the sonic vibrations imposed uponit. It is still not understood precisely why such selected resonantfrequencies produce the observed effects of drawing or forcing apenetrant liquid into a flaw when the surface opening of the flaw iscovered by the liquid and Why such sonic vibration will expel penetrantwhen the opening is no longer covered with liquid.

Example 1.--One specific example utilizing the discovered phenomenon isas follows: A large quantity of forged aluminum airframe fittings wereto be tested for flaws having surface openings. Suspected flaws in theforgings were closed forging tears. This type of flaw may be caused-by.a tear in the billet when it is subjected to the first or an earlyforging blow, which tear is then closed, but not welded, either insubsequent forming and shapingblows or later in theblow which caused thetear. It is difiicult to-locate this type of flaw, even withpainstaking, and precise fluorescent penetrant testing procedures asheretofore conducted. In order to determine a suitable resonant sonicfrequency for testing the number of forgings, a fewwere-tested by afluorescent penetrant procedure as disclosed in the above Ward patentuntila sample forging with an observable and known flaw with a surfaceopening was secured. The sample flawed forging was thenimmersed inabathof self-emulsifying fluorescent penetrant, such as disclosedin-the-said-Ward patent, and a polarized polycrystalline transducer(bariumtitanate) driven at its resonant frequency of 14,000 C. P. S. waspressed momentarily, i. e., about 10 seconds, against the. sampleforging while itwas immersed in the bath.. The particular frequency ofthe transducer was chosen because prior experience hadtaught theoperator that a part having the mass and shape of the forging was likelyto be resonant in the range of the chosen frequency. It should be notedthat the momentary immersion of the sample contrasted with therelatively prolongedimmersion of, say, fifteen minutes, allowed in priorfluorescent penetrant procedures for penetration of the flaw. The sampleforging was then removed from the penetrant bath, drained, and washed toremove from its surface the penetrant which still clung to it Theselected transducer was then applied after the sample had been dried.The second application of the transducer was made in relative darknessunder a black light for a momentary period of about twenty to thirtyseconds. A-fluores'cent indication began to appear as the faceplate ofthe transducer was moved over the surfaceof the sample forging,

but not as rapidly as it should have if the samplewere resonant at14,000 C. P. S. The sample wasreturned to the bath and the procedure wasrepeated, this time with a transducer vibratingat 15,500 C. P. S.Under'the black light a brilliant fluorescent indication ofthe flawbegan to appear in less than five seconds. To box in the resonanttransducer, the process was repeated'with' a transducer vibrating at18,250 C. P. S., but the appearance of the indication was not as rapidas when the 14,000 C. P. S. transducer was employed; If the'"15,50( C.P. S. transducer had not indicated the'reson'ant fre quency of theforging, one vibrating'at 12,750 C. P. S. would have been tried. In thisconnection it should be pointed out that as the resonant frequency ofanarti'cle increases, the band to which it will resonate appears tobroaden. Thus, at audible frequencies of 6,000 to 8,000 C. P. S., aresonant band may be as 'narr'owas 500 C. P. 8;, whereas at highultrasonic frequencies, say, above 1,000,000 C. P. S. the band maybeas'broadas 100,000 C. P. S. Ceramic transducers are recommended forthe purpose of'determining resonantfrequencies' of articles to be testedbecause of the universal range of ceramic material from audible to highultrasonic. Magneto-strictive transducers appear to be etficient onlybelow 50,000 C. P. S. But since most articles to be tested appear tohave a resonant frequence in this range, and since this process isespecially adapted for the production testing of mass produced articles,it has been found practical, once the resonant frequency of the articlehas been determined, to design magneto-strictive transducers for thearticle in order to take advantage of their ruggedness and the abilityto weld on a new face plate after a face plate has commenced to fatigueand pit in use.

Once the resonant frequency of the forgings had been determined, thetesting of individual pieces followed the procedure set forth in theabove Ward patent, except that the pieces were subjected to resonantVibrations in the bath of penetrant and after drying but prior toinspection under black light. Whereas about fifteen minutes would havebeen allowed for soaking in the bath, the pieces could be removed in amatter of seconds after being subjected to sonic vibrations. Likewise,while the pieces would have been normally allowed to stand after dryingfor about three-quarters of an hour to allow the indications to appear,subjecting the pieces to resonant frequencies for approximately thirtyseconds fully developed the indications. After sonic development of theindications, the pieces were dusted with finely powdered chalk. Whereassuch chalk is used in the Ward process to absorb penetrant from the flawopenings and provide a reflective background for the indications, inthis process the chalk simply provided a reflective background toenhance the brightness of the indications. When inspected under blacklight, the indications were noticeably bright because of the greateramount of exuded fluorescent penetrant and inspection was, therefore,much faster. It is suspected that this process developed indicationswhich would not have been developed without the aid of sonic vibrations,but the possible result is only speculative.

Example 2.-Iron castings were inspected with a selfemulsifying visiblepenetrant such as disclosed in the co-pending Ward and Switzerapplication, Serial No. 606,708, filed July 23, 1945, for Detection ofFlaws and according to the procedure taught therein, except that, priorto inspection, the castings were subjected to resonant sonic vibrations.The expected flaws were blowholes and shrinkage cracks at the filets,relatively open and deep flaws which are satisfactorily penetrated without the assistance of sonic vibration. The roughness of the surfacewarranted removal of the penetrant from the r surface of the castingswith a thorough scrubbing spray. Although this tended to removepenetrant from the flaw openings as well, such removal could betolerated because of the depth and volume of the flaws. The purpose ofsubjecting the washed and dried castings to resonant sonic vibrationswas to accelerate the appearance of the penetrant indications and toincrease their size and distinctiveness by expelling more penetrant fromthe flaws.

Example 3.-Ball-bearing races were inspected according to the procedureof the above Ward patent. The expected flaws were grinding cracks andpossibly cracks caused by heat-treatment. These cracks may be very fineand the parts normally require lengthy immersion to insure penetration.To accelerate penetration, the parts were subjected to a resonant sonicvibration. With the cracks fully filled with penetrant, they developeddistinctively and more quickly and a resonant sonic vibration was notdeemed necessary at the inspection stage.

Example 4.-T-hjs modification relied on the magnetostrictivecharacteristics of the alloy steel wrist pins being tested and is usefulonly for testing parts which are made of material having suchcharacteristics. Fine grinding and polishing cracks were the flaws mostlikely to be found in the wrist pins. While immersed in a bath offluorescent penetrant as disclosed in the above Ward patent, the pinswere pushed through a coil connected to a source of alternating currenthaving a frequency equal to the resonant frequency of the wrist pins. Asthe pins were pushed through the coil, they were subjected to pulses ofa high frequency magnetic field, the pulses being of a duration ofone-sixtieth of a second, spaced one-thirtieth of a second apart. Duringthese pulses or bursts, the pins became, in effect, the resonatingarmature of a magneto-strictive transducer. Penetration of the flaws bythe penetrant was greatly accelerated. The pins were then washed, driedand developed in the usual manner prior to inspection under black light.The development of the fluorescent flaw indications could have beenaccelerated by similarly subjecting the pins, after drying, to ahigh-frequency magnetic flux, but, as in Example 3 above, such aid tothe development of the flaws was not deemed necessary.

It is to be understood that in the foregoing examples, a non-fluorescentpenetrant may be employed instead of the stated fluorescent penetrant,or vice-verse. If a fluorescent penetrant is employed, it is composed ofa fluorescent dye dissolved in the penetrant vehicle to impart to thepenetrant a different fluorescent hue, or at least a hue of much greaterfluorescent intensity in thin films, than any actual or simulatednatural fluorescence of the vehicle, such a dye being hereafter referredto as a fiuoragent. If a non-fluorescent penetrant is employed, it iscomprised of a dissolved dye or finely dispersed pigment inseparablefrom the penetrant vehicle by the sonic vibrations employed andimparting to the penetrant a visible hue which contrasts in visiblelight from the hue of the background of the flaw indication; such anon-fluorescent dye or pigment is hereinafter referred to as a coloringagent. It is also to be understood that, to provide a light-reflectingbackground for the flaw indications provided by the exuded penetrant,one may apply to the test body a material which absorbs the penetrantand provides a contrasting, light-reflecting background for thefluoragent or coloring agent in the penetrant.

It should be apparent to those skilled in the art that the foregoingexamples are illustrative only and deal with test bodies having fairlyconstant cross-sections and, thus, fairly constant resonant frequenciesthroughout. The process may be operative for testing articles whichappear to have varying resonant frequencies in different portions. Intesting such articles it may be necessary to transduce differentfrequencies in different portions of the article, care being exercisedagainst fracturing the articles in the boundary zones between portionshaving different resonant frequencies.

As pointed out above, there are many possible speculative explanationsfor this process. At the resonant frequencies, the articles being testedmay stretch and shrink, thereby opening and closing the surface openingsand voids of the flaws and working the penetrant in and out as a liquidwould be worked in and out of the pores of a sponge. The highfrequencies of the vibrations may disturb the surface-active forces atthe interfaces of the flaws and the penetrant and thereby overcome orcounteract forces which might otherwise obstruct the movement of thepenetrant in the flaws. There is also some evidence that such sonicfrequencies radically alter the viscosities of liquids, creating minuteand fleeting cavitations in which the pressure is lower than the vaporpressure of the liquid, thereby causing the liquid to behave more likevapor than a liquid within the environs of.

such cavitations. Whenever more knowledge is gained in the art regardingthe correct explanation or explanations of the phenomenon involved,substantial improvements and changes may be made in this process withoutdeparting from the scope of this invention as set forth in the appendedclaims.

What'- is claimed is:

1. The process of ins'pecting' test bodies for flaws haw ing' surfaceopenings comprising the steps of applying a penetrant liquid to thesurface of the test body, removing' the penetrant from the surface,inspecting the body for indication of the location of such flaws by theappearance on the surface of the test body of a portion of the penetrantwhich was retained in the fiaw' during the removal of the penetrant fromthe surface, and subjecting' the test body to sonic vibrations to whichthe body is resonant after the penetrant has been removed from thesurface thereof in order to accelerate the appearance of flawindications.

v 2. The process of inspecting test bodies for flaws having surfaceopenings comprising the steps of applying a penetrant liquid to thesurface of the test body, removing the penetrant from the surface,inspecting the body for indication of the location of such flaws by theappearance on the surface of the test body of a portion of the penetrantwhich was retained in the flaw during the removal of the penetrant fromthe surface, and subjecting the test body to sonic vibrations to whichthe body is resonant when the penetrant is applied to the body in orderto accelerate the penetration of flaws by the penetrant.

3. The process as defined in claim 2 in which the body is also subjectedto sonic vibrations after the penetrant has been removed from thesurface thereof in order to accelerate the appearance of flawindications.

4. The process of inspecting test bodies of magnetostrictive materialfor flaws having surface openings comprising the steps of applying apenetrant liquid to the surface of the test body of magneto-strictivematerial, removing the penetrant from the surface, inspecting the bodyfor indication of the location of such flaws by the appearance on thesurface of the test body of a portion of the penetrant which wasretained in the flaw during the removal of the penetrant from thesurface, and subjecting the test body to sonic vibrations to which thebody is resonant by placing the body in magnetic field varying at afrequency substantially equal to the frequency at which said body isresonant.

5. The process as defined in claim 4 in which the test body is subjectedto said sonic vibrations after the penetrant has been removed from thesurface thereof in order to accelerate the appearance of flawindications.

6. The process as defined in claim 4 in which the test body is subjectedto said sonic vibrations when the penetrant is applied to the body.

7. The process as defined in claim 4 inv which the test body'issubjectedto said sonic vibrations when the penetrant is applied to the surfaceand after the pene trant has been removed.

8; The process of inspecting testbodies for flaws having surfaceopenings comprising the steps of applying a penetrant liquid containinga fiuoragent rendering the penetrant fluorescent to the surface of thetest body, removing the penetrant from the surface, inspecting the bodyunder fluoreseig'enous light for indication of the location of suchflaws by the appearance on the surface of the test body of a portion ofthe penetrant which was retained in the flaw during the removal of thepenetrant from the surface, and subjecting the test body to sonicvibrations to which the body is resonant.

9. The process as defined in claim 8 in which the surface of the testbody is provided with a light-reflecting background for the flawindications by applying a material which absorbs the penetrant andreflects light emitted by the fluoragent in the penetrant, said materialbeing applied before carrying out the inspection step.

10. The process of inspecting test bodies for flaws having surface"openings comprising the steps of applying a penetrant liquid containinga coloring agent of a color different from the surface color of the testbody to the surface of the test body, removing the penetrant from thesurface, inspecting the body under visible light for indication of thelocation of such flaws by the appearance on the surface of the test bodyof a portion of the penetrant which was retained in the flaw during theremoval of the penetrant from the surface, and subjecting the test bodyto sonic vibrations to which the body is resonant.

11. The process as defined in claim 10 in which the surface of the testbody is provided with a light-reflecting background for the flawindications by applying a material which absorbs the penetrant andreflects light of the hue of the coloring agent, said material beingapplied before carrying out the inspection step. 7

References Cited in the file of this patent UNITED STATES PATENTS DeForest et a1 May 11, 1954

